Autumn 2021 - Optical Connections Magazine



To meet the demands created by the expected 3.5 billion 5G users by 2026, carriers need to deploy fronthaul technology that can handle the high throughput of 5G. However, the wide range of applications and use cases have varying requirements, and there are many architectures that can serve each. This makes deciding on a fronthaul approach a challenging prospect because the market has no clear choice on which to converge, writes David Lewis , CTO Office, Lumentum.


and have different reach requirements in the fronthaul network. To address this, multiple WDM maps with multiple RAN partitioning architectures over duplex and bidirectional fibers are being applied. To keep costs down, 5G fronthaul connections are being designed using existing and mature optical technologies. These technologies were originally developed for highly cost- sensitive and high-volume data center

to six wavelengths in each direction. Low channel count and moderate reach WDM connections like these are pervasive in data center networking. In such cases, O-band (1260 to 1360 nm) wavelengths can be reused. A key advantage of this approach is that this reuse leverages the massive investments already made in maturing these O-band wavelengths for hyperscale data center interconnects. The three O-band contenders are coarse-, modified, and LAN- wavelength division multiplexing, also known as CWDM, MWDM, and LWDM respectively. CWDM is based on 20 nm spacing between channels with a 13 nm passband at the receiver. Such wide spacing allows for uncooled operation over a wide temperature range. Mobile network designers have added two wavelengths (1351 and 1371 nm) to the four wavelengths already used in data center networking: (1271, 1291, 1311 and 1331 nm). For architectures with separate up- and down-link fibers, six 25G eCPRI transceivers each operating on one of the CWDM wavelengths can be used together for an aggregate bandwidth of 150 Gb/s in each direction. For architectures with scarce fiber resources, MWDM achieves the same throughput as CWDM on a single

Fronthaul connectivity between active antenna units (AAU) utilizes either a distribution unit (DU) or a central unit (CU) that can operate over a distance up to ~ 10 km (see Figure 1). Spectrum allocations per carrier of 100 to 200 MHz mean that each AAU needs a bidirectional data rate of 25 to 50 Gb/s based on 1 or 2 ports of 25G eCPRI.

Figure 1: 5G fronthaul uses either a distribution unit (DU) or central unit (CU)

networking applications. Thus, 5G fronthaul equipment is being built on proven technologies that have already been cost-reduced through volume production. WAVELENGTH GRID CHOICES FOR 5G FRONTHAUL Many fronthaul

5G deployments aggregate the data from multiple AAUs resulting in 100’s of Gb/s of bidirectional data per DU/ CU over the fronthaul connections. As these fronthaul connections are often fiber constrained, wavelength division multiplexing (WDM) is used because it enables multiple AAUs to share the same fiber by allocating different wavelengths (channels) to the data of each AAU. In addition, the 5G rollout is happening in all the world’s regions, requiring network operators to decide how to architect the radio access network (RAN). Distributed RAN (D-RAN) and centralized RAN (C-RAN) architectures call for different numbers of channels

architectures connect three AAU base stations to a single distributed unit (DU), as shown in Figure 2. Such fronthaul networks can be realized with a small number of WDM channels, such as three

Figure 2: 5G fronthaul with 3 AAUs per DU


| ISSUE 25 | Q3 2021

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